Vinyl acetal polymers typified by vinyl butyral polymers are superior in toughness, film-forming property, dispersibility of inorganic and/or organic powders, etc., added, adhesiveness to a coated surface, and the like. Therefore, the vinyl acetal polymer has been used in a variety of intended usage such as intermediate films of laminated glasses, inks, paints, enamels for stoving, wash primers, lacquers, dispersants, adhesives, ceramic green sheets, thermally developable photosensitive materials, binders for aqueous ink-receiving layers, and the like. The principal reason for such use of the vinyl acetal polymer in the variety of intended usage is that the vinyl acetal polymer has superior toughness; however, to the contrary, the vinyl acetal polymer is accompanied by a disadvantage of inferior flexibility. Also, due to a high glass transition temperature, the vinyl acetal polymer is accompanied by a disadvantage of an inferior flow performance even under conditions of comparatively high temperatures.
In order to subject such a vinyl acetal polymer to a thermoplastic forming, it is necessary to add a plasticizer such as, e.g., a phthalic acid ester of an alcohol having 6 to 10 carbon atoms, triethylene glycol di(n-heptanoate), tri(2-ethylhexyl)phosphate, tricresyl phosphate or the like. However, compositions containing the vinyl acetal polymer and the plasticizer are likely to cause phase separation unless the plasticizer is appropriately selected, and is thus accompanied by a disadvantage of elution of the plasticizer into a solvent, leading to a remarkable change in properties of the composition. Therefore, films, sheets and the like obtained from the composition have unsatisfactory coating film strength, transparency, storage stability and the like, and an ink may leak out when printing is carried out on the surface thereof.
Accordingly, in order to prevent deterioration, etc., of various types of physical properties resulting from the addition of such a plasticizer, an attempt of internally plasticizing a vinyl acetal polymer has been conducted. For example, modified vinyl acetal polymers obtained by modifying a side chain of a vinyl alcohol polymer (hereinafter, may be also referred to as PVA) with polyalkylene oxide or the like and thereafter subjecting to acetalization have been known to be internally plasticized by a polyalkylene oxide structure of the side chain, whereby the flexibility is improved (see Japanese Unexamined Patent Application, Publication No. H06-263521). However, unreacted polyalkylene oxide remains in the resin according to this method; therefore, when films, sheets and the like are formed from the modified vinyl acetal polymer, they may get clouded. In addition, in a case in which the modified vinyl acetal polymer is used as a ceramic binder, there exist disadvantages such as: dispersibility of the ceramic powder in the modified vinyl acetal polymer being insufficient; phase separation of unreacted polyalkylene oxide during drying to remove the solvent for producing a green sheet, etc., thereby leading to a difficulty in obtaining a ceramic having high density; flexibility of the resultant green sheet or the like being insufficient; and the like. Furthermore, since the modified vinyl acetal polymer is inferior in storage stability, there also exists a disadvantage of time dependent elevation of the viscosity.
In addition, a vinyl acetal polymer produced by acetalizing PVA containing 1 to 20 mol % of ethylene at random, as a constitutional unit of the main chain has been proposed in Japanese Unexamined Patent Application, Publication No. 2003-183325. By introducing ethylene into the main chain in this manner, flexibility of the vinyl acetal polymer is improved, and the solution viscosity and the viscosity stability can be improved. However, since this vinyl acetal polymer is insufficient in solubility in a solvent of a sole alcohol, there exists a disadvantage of failure in meeting environmental concerns in recent years, i.e., the trend to a shift from use of mixed solvents containing an aromatic solvent such as toluene as a principal solvent to use of an alcohol solvent.
Furthermore, Japanese Unexamined Patent Application, Publication No. 2006-104309 discloses a polymer produced by acetalizing PVA including a 1,2-diol component in a side chain, as a vinyl acetal polymer accompanied by improved flexibility, lowering of the viscosity, etc., and also favorable viscosity stability. However, when films, sheets and the like are formed from this vinyl acetal polymer, a disadvantage of their coating film strength being insufficient may occur.
On the other hand, Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2008-504415 discloses, as a vinyl acetal polymer that is superior in plasticity, a vinyl acetal polymer formed by acetalizing PVA including a unit derived from a tertiary vinyl ester of an α-branched monocarboxylic acid having 9 to 15 carbon atoms. However, when the vinyl acetal polymer is used in a slurry composition for a ceramic green sheet, the vinyl acetal polymer has a disadvantage, for example, that handleability in printing, coating, or the like is inferior due to dispersibility in an inorganic dispersion medium being inferior, and thixotropic nature of the composition being insufficient, or the like.